The present invention relates generally to position sensors, and specifically to capacitive encoders of rotary or linear position.
Measurements of rotation angle and of linear displacement are widely used in various fields for the control of position, velocity and acceleration. Non-contact sensors used for these purposes are described generally in the first chapter of the Synchro/Resolver Conversion Handbook, Fourth Edition, published by DDC ILC Data Device Corp. (Bohemia, N.Y., 1994), which is incorporated herein by reference.
Commercially-available, non-contact, full-rotation transducers (commonly known as rotation angle encoders) are almost exclusively either optical shaft encoders or electromagnetic resolvers. Both of these types of transducers are well known in the art. They are sold both as integrated devices, which include their own shaft and bearings, and as modular devices, to be mounted on a host shaft.
Optical encoders provide binary level output signals and can be divided into absolute and incremental types. Encoders of the latter type are more popular, due to their flat construction and low cost, despite suffering from the following shortcomings:
Only relative position is measured.
Such encoders are sensitive to mechanical assembly and mounting errors.
The construction of such encoders affords only limited mechanical durability.
Absolute optical encoders are more expensive, more bulky and usually non-modular. In recent years, a modified absolute encoder was introduced, which provides sinusoidal, rather than binary-level, outputs, which can be interpolated to provide enhanced resolution.
Electromagnetic resolvers, which are described in detail in the above-mentioned Synchro/Resolver Conversion Handbook, are wound inductive components. They are relatively bulky and expensive, but highly durable. Single pole-pair resolvers provide two output voltages, which are proportional to sin xcex8 and to cos xcex8, wherein xcex8 is the rotation angle. Multi-pole-pair resolvers provide output voltages proportional to sin(nxcex8) and to cos(nxcex8), wherein n is the number of pole pairs. The resolution and accuracy of the multi-pole-pair resolver are high, but the output signals do not define the rotation angle unambiguously over a full rotation.
The two-speed resolver is equivalent to a combination of a single pole-pair and a multi-pole-pair resolver on the same shaft. It provides, simultaneously, two pairs of output voltages, which are referred to as coarse and fine channels. By processing both channels, an accurate and unambiguous reading is obtained. This kind of resolver, however, is even more bulky and expensive than its single or multi-pole-pair counterparts.
Linear optical encoders are incremental digital devices which, like incremental rotary encoders, include a reading head which moves relative to a ruler and generates output pulses. Currently, high-accuracy, long-stroke, linear encoders are almost exclusively of the optical type, although there are also some linear encoders based on magnetic principles. There is no capacitive linear encoder that is available commercially as a stand-alone component, but linear capacitive encoders are widely used in digital calipers.
In the context of the present patent application and in the claims, the term xe2x80x9cencoderxe2x80x9d refers to displacement transducers in which the interaction between the stationary and moving elements is based on a repetitive pattern, with a either binary or continuous output signal. The terms xe2x80x9cmoving elementxe2x80x9d and xe2x80x9crotorxe2x80x9d are used interchangeably with reference to rotary encoders, as are the terms xe2x80x9cstationary elementxe2x80x9d and xe2x80x9cstator.xe2x80x9d Likewise, the terms xe2x80x9creading headxe2x80x9d and xe2x80x9crulerxe2x80x9d refer respectively to the moving and stationary elements of linear encoders.
Even after many years of development, neither optical shaft encoders nor electromagnetic resolvers provide all of the following desirable features in combination:
Absolute reading with high accuracy and resolution.
Simple construction and low-profile packaging.
Low manufacturing costs.
Capacitive, full-rotation, absolute angle encoders (CFRAAEs) convert rotation angle into an output signal based on capacitive interaction between a rotor and a stator. They can be built to emulate the single-pole or multi-pole electromagnetic resolver, i.e., with an output signal that repeats once or more times per rotation, as well as multi-speed resolvers.
CFRAAEs, as described in the patent literature, would be expected to provide significant advantages over optical and inductive encoders. But CFRAAE devices have been entirely absent from the market as the result of a variety of difficulties, not all of which have been fully identified, appreciated, or solved. For example:
Accurate CFRAAE operation demands the discrimination of capacitances under one Femto-Farad (10xe2x88x9215 Farads) in the presence of parasitic capacitances and extraneous interference. Shielding against external interference is therefore of paramount importance.
It has been assumed that CFRAAEs requires costly, highly-accurate and stable electronic components. For example, German patent application DE 42 15 702 describes a capacitive angle encoder in which capacitances are individually corrected by laser trimming.
In CFRAAEs described in the patent literature, complex signal conditioning is required. Signal processing systems for use in this context are described, for example in German patent application DE 36 37 529 and in a corresponding U.S. Pat. No. 4,851,835, which is incorporated herein by reference.
There has been a lack of systematic classification and analysis of the various known encoder types. Consequently, novel configurations and possibilities for improvement have not been discovered.
It has therefore been the prevailing view in the field that a CFRAAE could not be commercially feasible. Only limited-rotation (substantially less than 360xc2x0) capacitive transducers have found practical use, and only in limited applications in which the transducer is integrated in a host system, mainly in optical mirror scanners. Typical limited-rotation transducers are described in U.S. Pat. Nos. 3,312,892, 3,732,553, 3,668,672, 5,099,386 and 4,864,295, which are incorporated herein by reference.
Analog full-rotation transducers, such as electromagnetic resolvers (in contrast with digital, or pulse-counting) transducers, typically provide two orthogonal output signals proportional to the sine and cosine of the rotation angle. Since capacitive coupling, unlike inductive coupling, is always positive, the only way, in general, to obtain a bipolar output in a capacitive transducer is to measure the difference between two displacement-dependent capacitances.
FIG. 1 is a typical schematic circuit diagram illustrating this principle (which is also applicable to capacitive linear displacement transducers). Two complementary excitation voltages Q and Qxe2x80x2 are applied to stationary transmitter plates 41 and 42, respectively. A moving receiver plate 40 is capacitively coupled to both transmitter plates and is connected to a charge amplifier 43, as is known in the art. The output voltage of the charge amplifier 43 is proportional to the difference of the respective capacitances C1 and C2 between receiver plate 40 and transmitter plates 41 and 42. The output of amplifier 43 is processed to provide the amplitude and polarity of the differential capacitance C1-C2, from which the position of plate 40 relative to plates 41 and 42 can be derived.
By analogy with electromagnetic resolvers, CFRAAEs can be made in both single-pole and multi-pole configurations. U.S. Pat. No. 5,598,153, which is incorporated herein by reference, describes a typical single-pole CFRAEE. French patent application 77 29354 describes a multi-pole encoder, in which the overlap between the rotor and a stator varies six times per revolution. The above-mentioned U.S. Pat. No. 4,851,835 describes an encoder in which a single rotor generates both coarse and fine signals.
Various methods are described in the relevant patent literature for converting a variable capacitance into output signal. The methods can be divided into two families:
1. Incorporating the variable capacitance in an oscillator circuit, which responds by varying its frequency, or duty cycle. Such methods are described, for example, in European patent application 0 459 118 A1; in German patent application DE 33 28 421; and in an article entitled xe2x80x9cKapacitives Sensorprinzip zur Absoluten Drehwinkelmessungxe2x80x9d (A capacitive sensor principle for absolute angle of rotation measurement), by Arnold and Heddergott, in Elektronpraxis (March, 1989).
2. Incorporating an AC excitation source for obtaining at least one AC or DC output signal which is a function of angle-dependent capacitances in the encoder. Two such outputs are required if a full rotation is to be covered. For example, U.S. Pat. No. 4,092,579, which is incorporated herein by reference, describes a capacitive resolver having one excitation voltage source and two output voltage signals proportional, respectively, to the sine and cosine of the rotation angle. U.S. Pat. No. 4,429,307, also incorporated herein by reference, describes a capacitive encoder with a similar circuit arrangement, except that two excitation voltages of opposite polarities are used.
Similar approaches are described, for example, in European patent application 0 226 716; in German patent application DE 36 37 529; and in an article entitled xe2x80x9cAn Accurate Low-Cost Capacitive Absolute Angular-Position Sensor with A Full-Circle Range,xe2x80x9d by Xiujun Li, et al., in IEEE Transactions on Instrumentation and Measurement, 45:2 (April, 1996), pp. 516-520.
The accuracy of such CFRAEE schemes based on AC excitation depends on the quality of the excitation voltages. Inaccuracies may result to the extent that the excitation signals are not of high harmonic purity and equal in amplitude, or if there is deviation from exact 90xc2x0 relative phase shift. The difficulties entailed can be overcome by circuit complexity, as illustrated by FIG. 3 in the above-mentioned German patent application DE 36 37 529. Solutions include complex digital emulation of the analog sinusoidal voltages, as proposed in European patent application 0 226 716, or employing accurate and stable analog circuit elements, as in the above-mentioned article by Li, et al.
German patent application DE 37 11 062 also describes a capacitive position measuring device using AC square wave excitation. The rotation angle is computed based on time sampling of a stepwise signal that results from interaction of the square wave excitation voltages with capacitance that varies with the rotor rotation (as shown in FIG. 2-d of that application). The disadvantage of such discrete sampling is an inferior signal-to-noise ratio (SNR), since sampling of the input voltage ignores its values between sampling times and is prone to noise.
Various methods are known in the art for electrically connecting the rotor and stator elements of a CFRAAE so as to provide the required transmission, rotation-dependent modulation, and sensing of an electrostatic field in the encoder. For example, U.S. Pat. Nos. 3,873,916 and 4,404,560, which are incorporated herein by reference, have the general electrical configuration of FIG. 1, wherein transmitting plates 41 and 42 represent the stator, and receiving plate 40 represents the rotor. This configuration is problematic in that the rotor must be electrically connected to the processing electronics. To enable free rotation, a slip ring, with its known disadvantages of friction and unreliability, must be used to make the electrical connection.
Other encoder types have both the electrostatic field transmitter and the receiver located on one or more stator elements. For example, U.S. Pat. No. 5,099,386, which is incorporated herein by reference, describes an encoder which has a dielectrically-patterned, non-conductive rotor between transmitting and receiving stators. There is thus no need for electrical connection to the rotor, but the rotation angle is limited.
U.S. Pat. Nos. 3,668,672 and 3,732,553, which are incorporated herein by reference, describe CFRAAEs of generally similar construction to that in U.S. Pat. No. 5,099,386, except that the rotor has a patterned conductive coating. The coating is electrically grounded and serves as an electrical shield that selectively varies the measured capacitance between the stator elements. Various methods have been proposed for grounding the rotor. For example, FIG. 10 of U.S. Pat. No. 3,668,672 shows slip rings in use for this purpose. This approach has the disadvantages of friction, low reliability, and high cost. U.S. Pat. No. 3,732,553 provides rotor grounding by relying on the contact between the grounded encoder housing and the rotor through a shaft to which the rotor is fixed, but this contact can also be problematic, as described further hereinbelow.
European patent application 0 459 118 illustrates (in FIG. 2 thereof) a contacting tip used to ground a rotor. This approach suffers from similar disadvantages to those of the U.S. patents cited above. Similarly, grounded rotors are used in capacitive encoders described in the above-mentioned article by Li, et al., and in an article entitled xe2x80x9cAn Integrable Capacitive Angular Displacement Sensor with Improved Linearity,xe2x80x9d by Wolffenbuttel and Van Kampen, in Sensors and Actuators A.25-27 (1991), pp. 835-843.
Another type of CFRAAE has conductive-coatings on both sides of the rotor, which are electrically interconnected but otherwise floating. If at least one coating is patterned, then the rotor serves as an angle-dependent coupling bridge between transmitter and receiver plates on the stators. Encoders of this type are described in U.S. Pat. Nos. 3,845,377, 3,312,892, 4,092,579, 4,851,835, 4,238,781 and 4,788,546, which are incorporated herein by reference, and in German patent application DE 42 15 702. In U.S. Pat. No. 4,851,835, one side of a multi-pole rotor pattern is separated into multiple individual elements.
In another, xe2x80x9cfoldbackxe2x80x9d configuration, described in U.S. Pat. Nos. 3,961,318 and 4,429,307, which are incorporated herein by reference, the transmitter and receiver plates are placed on a common stationary substrate of a single stator on one side of the rotor and are coupled by the conductive pattern on the rotor. Alternatively, in a symmetrical version of this type of encoder, two stator elements, each with its own transmitter and receiver plates, are placed on either side of the rotor. CFRAAEs of this type are described in U.S. Pat. No. 4,788,546, which is incorporated herein by reference, as well as in German patent application DE 37 11 062, and U.K. patent application GB 2 176 013.
Capacitive linear displacement encoders (CLDEs) are also known in the art, but have used only a portion of the possible topologies suggested in the CFRAAE literature. For example, U.S. Pat. No. 4,429,307, which is incorporated herein by reference, describes a CLDE having a head that includes two sinusoidal conductive patterns, which are excited by two complementary excitation voltages. The voltages are generated on a ruler and capacitively coupled to the head via coupling strips, or transmitter plates. The patterns on the head couple back to sine and cosine receiver plates on the ruler. The moving head is thus capacitively coupled to the ruler and does not need electrical wiring. There is no mention in the patent as to how the receiver plates are protected from external interference and how direct coupling from the coupling strips to the receiver plates is eliminated. Also, since the gains of the sine and cosine channels depend on the air gaps of their respective coupling strips, any difference between the air gaps will affect the relative gain. The accuracy is therefore sensitive to tilt between the head and ruler and necessitates very stable and accurate electronic components.
U.S. Pat. No. 3,961,318, which is incorporated herein by reference, describes two different versions of Type 5 CLDEs. In the first version, the ruler is electrically unwired and capacitively coupled to the head, which includes both excitation sources and the receiver. The ruler electrodes can be segmented and isolated from each other, so as to enable several rulers to be butted together without electrical interconnection, and thus to extend the measurement range. The second version is similar to that in the above-mentioned U.S. Pat. No. 4,429,307. As in that patent, the ratio of the sine and cosine signals is sensitive to both tilt and component tolerance, and no answer is provided to problems of parasitic capacitive coupling between the adjacent transmitter and receiver plates or of protection against external interference.
U.S. Pat. No. 4,586,260, which is incorporated herein by reference, describes a digital Vernier caliper that employs a capacitive linear encoder. This encoder is further described in Chapter 18 of Capacitive Sensors, by Larry K. Baxter (IEEE Press, 1997). The excitation is provided by means of two complementary square waves, and the head is divided into sine and cosine portions, each including is own receiver plate and amplifier. The ruler is unwired. The disadvantages of this CLDE are as follows:
1. Since there are two signal channels, separated both spatially and electronically, their gain matching is sensitive to head tilt and to electronic component tolerance and temperature stability.
2. The ruler pattern is rectangular rather than sinusoidal. The resulting spatial harmonics of the pattern, although largely suppressed by the air gap, limit the accuracy obtainable by interpolation.
3. There are gaps between individual ruler elements that are perpendicular to the direction of motion and add further spatial harmonics.
High-accuracy CLDE devices known in the art are incremental, i.e., they give a relative, rather than absolute, displacement reading. On the other hand, U.S. Pat. No. 3,312,892, which is incorporated herein by reference, describes a capacitive displacement transducer which is based on the overlap between triangular stationary plates and a rectangular moving plate. This configuration constitutes an absolute, but essentially xe2x80x9ccoarse,xe2x80x9d encoder. SUMMARY OF THE INVENTION
It is an object of the present invention to provide improved devices and methods for capacitive position sensing.
It is an object of some aspects of the present invention to provide improved capacitive rotation angle encoders, and particularly full rotation absolute angle encoders.
It is an object of other aspects of the present invention to provide improved capacitive linear displacement encoders.
It is yet a further object of some aspects of the present invention to provide compact, self-contained capacitive position encoders.
It is still a further object of some aspects of the present invention to provide encoders that exhibit improved accuracy and reduced sensitivity to external interference and environmental conditions.
In preferred embodiments of the present invention, a capacitive motion encoder, for sensing the position of a moving object relative to a stationary object, includes at least one stationary element, coupled to the stationary object, and a moving element, coupled to the moving object. A periodic (time-modulated) electrostatic field is transmitted by a transmitter plate, which is preferably on the stationary element, but may alternatively be on the moving element. An electrically active pattern on one of the elements, typically on the moving element, modulates the envelope of the time-modulated electrostatic field responsive to motion of the moving object. The pattern preferably comprises conductive material plated on the element, although methods of creating patterned dielectric moving element may also be used. The term xe2x80x9celectrically active,xe2x80x9d as used in the context of the present patent and in the claims, may refer to any such pattern. Processing circuitry senses the modulated electrostatic field and analyzes the envelope modulation to determine a measure of the position of the moving object.
The moving and stationary elements are substantially enclosed by a conductive shield, which is electrically decoupled from both the moving and the stationary objects, and which shields the elements from electrical interference. The inventor has found, in distinction to capacitive position sensors known in the art, that separating the shield from other objects around the encoder affords superior protection of the inherently-low signal levels in the encoder against both external interference and parasitic coupling to excitation voltages supplied to generate the electrostatic field and operate the encoder. Preferably, the conductive shield encloses the processing circuitry, as well as the moving and stationary elements.
In some preferred embodiments of the present invention, the encoder comprises a rotation angle encoder, preferably a full rotation absolute angle encoder. In these embodiments, the moving element comprises a rotor, the moving object comprises a rotating shaft, and the at least one stationary element comprises one or more stators, such that the processing circuitry determines a measure of the rotational position of the shaft. The shield and rotor preferably have a labyrinthine configuration in a region in which the rotor is attached to the shaft, so as to prevent leakage of electrical interference into the shield.
In other preferred embodiments of the present invention, the encoder comprises a linear displacement encoder. In these embodiments, the stationary element preferably comprises a linear ruler, which may be too long to be practically enclosed by the shield. In this case, the shield preferably encloses the moving element and a portion of the stationary element over which the moving element is positioned at any given time.
In some preferred embodiments of the present invention, a capacitive rotation angle encoder for sensing position of a rotating shaft comprises a transmitter and a receiver, typically in the form of transmitting and receiving plates on one or more stators. The transmitter is made up of multiple segments disposed about the shaft, each of which generates a periodic electrostatic field at a common frequency, but having a different, predetermined phase from the other segments. Preferably, four segments are used for each one or more poles on the rotor. The transmitter segments are excited with AC voltages, which are in mutual quadrature. The resulting fields are modulated by rotation of a rotor, and the modulated fields are received by the receiver.
Processing circuitry associated with the encoder comprises two synchronous detector circuits, which receive periodic inputs from two, respective transmitter excitation sources and process signals from the receiver in synchronization with the generated field so as to generate outputs indicative of the sine and cosine of the rotation angle. Preferably, the detector circuits follow a single charge amplifier through which the signals due to all of the transmitter segments are received for processing. The use of such phase/quadrature excitation (PQE) and synchronous detection enables the angle of the rotor to be determined with greater accuracy, better signal-to-noise ratio and reduced sensitivity to deviation in component values than is achieved by encoders known in the art, which typically use signal sampling, rather than full synchronous detection, or employ more than one signal processing channel, rather than the single charge amplifier of the present invention. Consequently, neither high-stability, precision circuitry nor component trimming is required. The resulting simple circuitry can be conveniently packaged together with the rotor and stator(s) and protected within the conductive shield.
Alternatively, the encoder comprises a unitary transmitter and segmented receiver plates, and outputs sine and cosine signals as described above. In one of these preferred embodiments, the encoder further comprises a rectifier, which rectifies an AC input to the encoder so as to provide DC voltage to the detector circuit, so that the encoder can be conveniently substituted for an inductive resolver.
In other preferred embodiments of the present invention, the principles of phase/quadrature excitation are applied to impart similar advantages to a capacitive linear encoder.
In some preferred embodiments of the present invention, the encoder comprises a multi-speed encoder, wherein the electrically-active pattern comprises smoothly-varying, coarse and fine periodic electrically-active patterns, which are preferably sinusoidal in shape. The coarse and fine patterns have respective low and high spatial frequencies as a function of position on the element on which the patterns are formed. As the moving element moves, the patterns modulate an envelope of the electrostatic field in accordance with the low and high spatial frequencies. The processing circuitry senses the modulation so as to determine coarse- and fine-resolution measures of the position of the moving object. The demodulated outputs of the processing circuitry are very accurate because of the smooth variation of the patterns, unlike multi-speed encoders known in the art. Preferably, the coarse measure is an absolute measure of rotational or linear position.
In one of these preferred embodiments, the moving element further has an intermediate electrically active pattern thereon, having a spatial frequency between the high and low frequencies. The processing circuitry senses modulation of the field corresponding to the intermediate pattern, as well, in order to determine a measure of the position of the moving object at a resolution intermediate the coarse and fine measures.
In some of these multi-speed embodiments, the coarse pattern is divided into a plurality of segments, distributed over a surface of the moving element so as to reduce variations arising in the modulation of the field due to tilt of the rotor relative to the stator. Segmentation of the pattern for this purposed may also be applied in single-speed embodiments of the present invention.
In some preferred embodiments of the present invention in which the encoder comprises coarse and fine patterns, the processing circuitry switches the electrostatic field so that it is modulated alternately by the two patterns. The circuitry thus alternately determines coarse and fine measures of the position of the moving object. The switching is preferably performed by alternately exciting different transmitting regions of the stationary element. By virtue of switching between the regions in this manner, both the coarse and fine measurements can be made using a single stationary element and a single moving element, typically a single stator and a single rotor, and without duplication in the signal processing circuitry.
In other preferred embodiments of the present invention, the rotor of a capacitive rotation angle encoder has an electrically-active pattern, which repeats multiple times about the shaft at a predetermined angular frequency and is also characterized by a rotational asymmetry, such as an eccentricity. The processing circuitry senses modulation of the electrostatic field due to the pattern so as to determine a fine measure of the rotation angle, and also sense modulation due to the eccentricity in order to determine a coarse measure of the rotation angle of the shaft. There is thus no need for a separate coarse pattern and detection channel, as in the preceding preferred embodiments, so that the center hole of the encoderxe2x80x94in the case of a hollow-shaft encoderxe2x80x94can be made relatively larger to accommodate larger shaft sizes.
In some preferred embodiments of the present invention, the moving element comprises a plurality of electrically-active segments, which are mutually separated by open spaces in the substrate to eliminate moisture film effects. The segments may or may not be electrically insulated. Preferably, the moving element comprises a rotor, wherein the electrically active segments protrude radially outward around the shaft. The use of electrically and mechanically separated segments both reduces the sensitivity of the encoder to tilt and prevents moisture accumulation from affecting the reading. Although some capacitive encoders known in the art use three-dimensional electrically-active patterns, none are open in the manner of the present invention.
In some preferred embodiments of the present invention, first and second stationary elements are disposed on opposing sides of the moving element so as to transmit an electrostatic field therethrough. A potential stabilization circuit maintains the moving element at a steady, virtually-grounded potential by sensing AC potential at the first stationary element and applying and opposite potential to the second stationary element. No physical or electrical contact with the moving element is required. Grounding the moving element is known in the art to be advantageous in certain types of encoders. Unlike the present invention, however, prior art encoders of such types require contact to be made with the moving element, using a slip ring, for example, in order to ground it.
In some preferred embodiments of the present invention, the encoder comprises a linear displacement encoder, wherein the stationary element comprises a ruler, and the moving element comprises a reading head that travels along the ruler. In some of these preferred embodiments, the transmitter and receiver plates are on the head, and the electrically-active pattern with which the plates interact is on the ruler. In other embodiments, the transmitter plates are on the ruler, and the receiver pattern is on the head.
In some of these preferred embodiments, the ruler comprises a flexible printed circuit material, which is fixed along a surface of the stationary object, for example, on a machine that is controlled using the encoder. The surface may be flat or curved. In one such preferred embodiment, the stationary object is generally cylindrical, and the encoder is used to measure angle around an axis of the cylinder.
Preferably, the pattern on the ruler or on the head is designed for improved stability and accuracy of measurement by comparison with capacitive linear encoders known in the art, particularly in terms of immunity to variations in alignment, angle and spacing between the reading head and ruler, as well as immunity to external interference and humidity. Preferably, the pattern is symmetrical with respect to relative tilt of the head and ruler in both up/down and side-to-side directions, so as to reduce tilt sensitivity. Further preferably, the pattern is intermittently broken by gaps, particularly in embodiments in which the pattern is on the ruler, so as to prevent coupling of interference by the pattern into the reading head. Preferably, phase/quadrature excitation is applied to the transmitter plates, and the output of a single receiving plate and amplifier associated therewith is processed using synchronous detection, as described hereinabove.
In some preferred embodiments of the present invention, the capacitive linear displacement encoder provides an absolute position measurement. Preferably, the measurement is made by alternately sensing fine and coarse patterns on the reading head or on the ruler. Alternatively or additionally, an index is provided at one end of the ruler, and an index position of the reading head is sensed using the index in order to provide an absolute reference position for subsequent incremental measurements. An index may similarly be provided in rotation angle encoders in accordance with preferred embodiments of the present invention.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
at least one stationary element, coupled to the stationary object;
a moving element, coupled to the moving object and in proximity to the stationary element;
a field transmitter, which generates an electrostatic field, which is modulated by a change in capacitance between the stationary and moving elements responsive to relative motion of the elements;
a conductive shield, which is electrically decoupled from both the moving and the stationary objects, and which encloses the moving and stationary elements so as to shield the elements from external electrical interference; and
processing circuitry, coupled to sense the modulated electrostatic field and to determine responsive thereto a measure of the position of the moving object.
Preferably, the moving element includes a rotor, and the moving object includes a rotating shaft, and wherein the at least one stationary element includes at least one stator, such that the processing circuitry determines a measure of the rotational position of the shaft. Further preferably, the conductive shield encloses at least a portion of the processing circuitry together with the rotor and the at least one stator. Most preferably, the at least one stator and the rotor include printed circuit boards, on at least one of which at least the portion of the processing circuitry is mounted.
Preferably, the rotor includes a generally planar plate and a substantially non-planar, annular hub for coupling the rotor to the shaft, and the shield extends into the plane of the rotor adjacent to the hub so as to prevent electrical interference from passing from the shaft to the rotor. Preferably, the encoder is configured so that the rotor can rotate by at least 360xc2x0 relative to the stator.
In a preferred embodiment, the encoder includes a mechanical housing around the moving and stationary elements, which housing is electrically decoupled from the shield. Preferably, the at least one stationary element includes two generally parallel, mutually spaced stationary elements, one including the field transmitter and the other including a field receiver, which are electrically coupled one to the other in the housing by pressure of the elements against a flexible conductive member therebetween.
Preferably, the stationary element includes a printed circuit board including an extension which protrudes through the shield, to which an electrical connection is made to the encoder.
In a preferred embodiment, the field transmitter is attached to the stationary element and is coupled so as to form a part of the conductive shield.
Preferably, the field transmitter is attached to the stationary element, and the moving element has an electrically-active pattern thereon, which modulates the electrostatic field. Preferably, the electrically-active pattern includes a conductive material or, alternatively or additionally, a dielectric material. Preferably, the at least one stationary element includes a receiver of the electrostatic field, which is coupled to the processing circuitry. Preferably, the conductive, electrically-active pattern on the moving element is held at a generally constant potential. Alternatively, the conductive, electrically-active pattern on the moving element is electrically floating. In a preferred embodiment, the at least one stationary element includes a single element to which both the transmitter and receiver are attached. In another preferred embodiment, the encoder includes a second stationary element having both a transmitter and receiver attached thereto.
Alternatively, the conductive material on the moving element is coupled to the processing circuitry and serves as a receiver of the electrostatic field.
There is also provided, in accordance with a preferred embodiment of the present invention, a capacitive angle encoder for sensing position of a rotating shaft, including:
a transmitter, including multiple segments disposed about the shaft, each segment generating a periodic electrostatic field at a common frequency, but having a different, predetermined phase from the other segments;
a receiver, which generates signals responsive to the fields from the multiple segments such that the strength of reception of each of the fields is modulated by a variation of a capacitance between the transmitter and the receiver as a function of rotation of the shaft; and
a detector circuit including at least one synchronous detector, which processes the signals in synchronization with the generated field so as to generate an output indicative of the rotation angle.
Preferably, the at least one synchronous detector includes two synchronous detectors, which generate respective outputs proportional to the sine and cosine of the rotation angle. Further preferably, the receiver includes a single input amplification channel through which the signals from all of the transmitter segments are received for processing.
Preferably, the transmitter plate includes a generally planar transmitting stator, divided into multiple segments disposed around an axis of the shaft, wherein the multiple segments are most preferably arranged in four quadrants.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a capacitive angle resolver for sensing position of a rotating shaft, including:
a transmitter, which generates a periodic electrostatic field responsive to an AC electrical input at a given frequency;
a receiver, including multiple segments disposed about the shaft, which generate signals responsive to the field from the transmitter such that the field received at each of the segments is modulated by a variation of a capacitance between the transmitter and the receiver as a function of rotation of the shaft;
a signal processing circuit, which processes the signals from the receiver segments so as to generate an AC output indicative of the rotation angle; and
a rectifier circuit, which rectifies the AC input so as to provide DC voltage to the detector circuit.
Preferably, the encoder includes a rotor coupled to rotate with the shaft and having an electrically active pattern thereon, such that rotation of the rotor modulates the field received at the receiver. Further preferably, the electrically active pattern includes conductive material, which is most preferably held at a ground potential.
There is further provided, in accordance with a preferred embodiment of the present invention, a capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
at least one stationary element, coupled to the stationary object;
a moving element, coupled to the moving object and in proximity to the stationary element;
an electrostatic field transmitter, associated with one of the stationary or moving elements;
a field modulator associated with another of the stationary and moving elements, including smoothly-varying coarse and fine periodic electrically-active patterns on the element, the patterns varying along a dimension of the element with respective low and high spatial frequencies, which modulate the electrostatic field by inducing a variation in capacitance between the stationary and moving elements responsive to relative motion of the elements, at modulation frequencies corresponding to the low and high spatial frequencies, substantially without spatial harmonics thereof; and
processing circuitry, coupled to sense the modulated electrostatic field and to determine responsive thereto coarse and fine measures of the position of the moving object.
Preferably, the stationary element includes stator, made of a single planar element including a plurality of conductive areas, at least one of which is the field transmitter, and another of which receives the field.
Further preferably, the moving element includes a rotor, on which the electrically-active patterns are formed such that the coarse periodic pattern includes a generally circular area on the rotor, disposed eccentrically about the shaft, and the fine periodic pattern includes a sinusoidal pattern on the rotor, disposed circumferentially around the shaft.
In a preferred embodiment, the field modulator further includes an intermediate electrically active pattern thereon, having a spatial frequency intermediate the high and low frequencies, and the processing circuitry senses modulation of the field corresponding to the intermediate frequency.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
at least one stationary element, coupled to the stationary object;
a moving element, coupled to the moving object and in proximity to the stationary element;
an electrostatic field transmitter, associated with one of the stationary or moving elements;
a field modulator associated with another of the stationary and moving elements, including coarse and fine, periodic, electrically-active patterns on the element, the patterns varying along a dimension of the element with respective low and high spatial frequencies, which modulate the electrostatic field by inducing a variation in capacitance between the stationary and moving elements responsive to relative motion of the elements, at modulation frequencies corresponding to the low and high spatial frequencies; and
processing circuitry, which switches the electrostatic field so that it is modulated alternately by the coarse or by the fine pattern, and which senses the modulated field so as to alternately determine, responsive thereto, coarse and fine measures of the position of the moving object.
There is in addition provided, in accordance with a preferred embodiment of the present invention, a capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
a stationary element, coupled to the stationary object, and including an electrostatic field transmitter and receiver;
a moving element, coupled to the moving object and in proximity to the stationary element, and including a field modulator including coarse and fine periodic electrically-active patterns on the moving element, which vary along a dimension of the element with respective low and high spatial frequencies, and which modulate the electrostatic field by inducing a variation in capacitance between the stationary and moving elements responsive to relative motion of the elements, at modulation frequencies corresponding to the low and high spatial frequencies; and
processing circuitry, coupled to sense the modulated electrostatic field and to determine responsive thereto coarse and fine measures of the position of the moving object.
There is also provided, in accordance with a preferred embodiment of the present invention, a capacitive angle encoder for sensing the position of a rotating shaft, including:
one or more stators, one of which includes a field transmitter which generates an electrostatic field;
a rotor, coupled to rotate with the shaft, and having a rotationally-asymmetric electrically-active pattern thereon which modulates the electrostatic field responsive to rotation of the shaft by inducing a variation in capacitance between the stator and the rotor that repeats once for each rotation of the shaft, wherein the pattern is divided into a plurality of sub-areas so as to reduce variations arising in the modulation of the field due to tilt of the rotor relative to the stator; and
processing circuitry, coupled to sense the modulated electrostatic field and to determine, responsive to the coarse and fine variations, coarse and fine measures of the angle of the shaft.
Preferably, the electrically-active pattern on the rotor includes a generally circular area located eccentrically relative to an axis of the shaft.
There is still further provided, in accordance with a preferred embodiment of the present invention, a capacitive angle encoder for sensing position of a rotating shaft, including:
a rotor, coupled to the shaft, having an electrically-active region which has a rotational asymmetry about an axis of the shaft and includes a pattern disposed circumferentially on the rotor which repeats multiple times about the shaft at a predetermined angular frequency;
at least one stator, having an electrostatic field associated therewith, which is modulated due to a variation in capacitance induced by the electrically active region due to rotation of the rotor; and
processing circuitry, which senses modulation of the field occurring once per rotation of the shaft due to the rotational asymmetry of the region so as to determine, responsive thereto, a coarse measure of the rotation angle of the shaft, and which senses modulation of the field due to the pattern so as to determine a fine measure of the rotation angle.
In a preferred embodiment, the encoder includes a receiver coupled to one of the one or more stators and characterized by a rotational asymmetry relative to the shaft axis such that the coarse measure of the angle is determined responsive to an interaction between the asymmetries of the fine pattern and the receiver.
In another preferred embodiment, the encoder includes a switch, which is actuated to determine alternately the coarse and fine measures of the angle.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, a moisture-resistant capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
at least one stationary element, coupled to the stationary object, and having an electrostatic field associated therewith;
a moving element, coupled to the moving object, including a plurality of electrically active segments mutually separated by spaces, the segments forming a pattern that modulates the electrostatic field due to a variation in capacitance between the stationary and moving elements as the moving element moves; and
processing circuitry, coupled to sense the modulated electrostatic field and to determine responsive thereto a measure of the position of the moving object.
Preferably, fluid impinging on the moving element is displaced from the segments to the spaces separating the segments.
In a preferred embodiment, the moving object includes a rotating shaft, and the moving element includes a rotor from which the electrically active segments protrude radially outward around the shaft, and the at least one stationary element includes at least one stator, such that the processing circuitry determines a measure of the rotational position of the shaft.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
a moving element, coupled to the moving object, having an electrically-active pattern thereon;
first and second stationary elements, coupled to the stationary object, disposed on opposing sides of the moving element so as to transmit an alternating electrostatic field therethrough;
processing circuitry, coupled to sense modulation of the electrostatic field responsive to a variation in capacitance between the stationary elements due to movement of the electrically-active pattern therein and to determine responsive thereto a measure of the position of the moving object; and
a potential stabilization circuit, which maintains the moving element at a generally constant potential by sensing an alternating electrical potential at the first stationary element and applying an opposite potential to the second stationary element.
Preferably, the generally constant potential includes a virtual ground. Further preferably, the potential stabilization circuit makes substantially no electrical contact with the moving element.
There is yet further provided, in accordance with a preferred embodiment of the present invention, a capacitive linear displacement encoder, for sensing the position of a moving object relative to a stationary object, including:
a ruler, fixed to the stationary object;
a reading head, fixed to the moving object so as to move along the ruler, and including an electrostatic field transmitter, which generates an electrostatic field in a vicinity of the reading head;
an electrically-active pattern formed on the ruler, which pattern causes a variation in capacitance between the ruler and reading head so as to modulate the electrostatic field responsive to motion of the reading head relative to the ruler, the pattern having a symmetry such that the modulation is substantially unaffected by tilt of the head relative to the ruler; and
processing circuitry, coupled to sense the modulated electrostatic field, so as to detect the modulation and determine responsive thereto a measure of the position of the moving object.
Preferably, the pattern includes a double sinusoid. Further preferably, the reading head includes a receiver, which receives the modulated electrostatic field, and the pattern is intermittently broken by gaps in the pattern, so as to inhibit coupling of interference along the pattern into the reading head, wherein the gaps are formed at an acute angle relative to a longitudinal axis of the ruler.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a capacitive linear displacement encoder, for sensing the position of a moving object relative to a stationary object, including:
a ruler, fixed to the stationary object, and including an electrostatic field transmitter, which generates an electrostatic field in a vicinity of the ruler;
a reading head, fixed to the moving object so as to move along the ruler and having an electrically-active pattern formed thereon, which pattern causes a variation in capacitance between the ruler and reading head so as to modulate the electrostatic field responsive to motion of the reading head relative to the ruler, the pattern having a symmetry such that the modulation is substantially unaffected by tilt of the head relative to the ruler; and
processing circuitry, coupled to sense the modulated electrostatic field, so as to detect the modulation and determines responsive thereto a measure of the position of the moving object.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a capacitive linear displacement encoder, for sensing the position of a moving object relative to a stationary object, including:
a ruler, fixed to a curved surface of the stationary object;
a reading head, fixed to the moving object so as to move along the ruler;
an electrostatic field transmitter, which generates an electrostatic field in a vicinity of the reading head;
an electrically-active pattern formed on the ruler or the reading head, which pattern causes a variation in capacitance between the ruler and the reading head so as to modulate the electrostatic field responsive to motion of the reading head relative to the ruler; and
processing circuitry, coupled to sense the modulated electrostatic field, so as to detect the modulation and determines responsive thereto a measure of the position of the moving object along the curved surface.
In a preferred embodiment, the stationary object has a generally cylindrical form, and wherein the measure of the position of the moving object includes an angular measurement about an axis of the stationary object.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, a capacitive linear displacement encoder, for sensing the position of a moving object relative to a stationary object, including:
a ruler, fixed to the stationary object;
a reading head, fixed to the moving object so as to move along the ruler;
transmitting plates fixed to the ruler, so as to generate and receive an electrostatic field in a vicinity of the reading head, the plates having coarse and fine reading configurations;
an electrically-active receiving plate on the reading head, the plate configured such that motion of the head relative to the ruler causes a variation in capacitance between the transmitting and receiving plates, which modulates the electrostatic field received by the receiving plate; and
processing circuitry, coupled to sense the modulated electrostatic field, so as to detect the modulation of the field in the coarse reading configuration so as to determine responsive thereto a coarse measure of the position of the moving object, and to detect the modulation of the field in the fine reading configuration so as to determine responsive thereto a fine measure of the position of the moving object.
Preferably, the coarse measure includes an absolute position measurement.
In a preferred embodiment, the transmitting plate includes a plurality of transmitting bars, which are collectively divided into at least two triangular regions, and wherein in the coarse configuration the bars in each of the regions are collectively excited. Preferably, the receiving plate includes a conductive, periodic pattern superimposed on a generally quadrilateral region, and wherein when the transmitting plate is operating in the coarse reading configuration, the entire quadrilateral region is held at a common electrical potential.
There is also provided, in accordance with a preferred embodiment of the present invention, a capacitive motion encoder for sensing the position of a moving object relative to a stationary object, including:
at least one stationary element, coupled to the stationary object;
a moving element, coupled to the moving object;
transmitting and receiving plates fixed to the stationary or the moving element, so as to generate and receive an electrostatic field in a vicinity of the moving element, the plates including at least one index plate at an index position on the stationary element, such that the electrostatic field encountered by the moving element while in proximity to the at least one index plate is identifiably different from that at other locations along the stationary element;
an electrically-active pattern formed on one of the elements, which pattern causes a variation in capacitance between the elements so as to modulate the electrostatic field responsive to motion of the moving element relative to the stationary element; and
processing circuitry, coupled to sense the modulated electrostatic field, and to identify the difference in the field when the moving element is in proximity to the index plate so as to determine responsive thereto that the moving element is in the index position, and which detects the modulation and determines responsive thereto a measure of the position of the moving object relative to the index position.
There is still further provided, in accordance with a preferred embodiment of the present invention, a method for sensing position of a rotating shaft, including:
transmitting periodic electrostatic fields having a common frequency at a plurality of angular locations around the shaft, each field having a different, predetermined phase from the other signals;
sensing the fields from the plurality of locations, and generating signals responsive to modulation of the fields engendered due to a variation in capacitance as a function of rotation of the shaft; and
processing the signals in synchronization with the frequency of the transmitted fields so as to generate outputs indicative of the rotation angle.
There is yet further provided, in accordance with a preferred embodiment of the present invention, a method for sensing position of a rotating shaft, including:
receiving an AC electrical input at a given frequency;
generating a periodic electrostatic field responsive to the AC input;
rectifying a portion of the AC input so as to provide DC voltage to a detector circuit;
sensing the field at a plurality of locations, and generating signals responsive to modulation of the fields engendered due to a variation in capacitance as a function of rotation of the shaft; and
processing the signals using the detector circuit so as to generate an AC output at the given frequency indicative of the rotation angle.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for sensing the position of a moving object relative to a stationary object, including:
transmitting an electrostatic field in a vicinity of the moving object;
associating smoothly-varying coarse and fine periodic electrically-active patterns with the moving object, the patterns varying along a dimension of motion of the object with respective low and high spatial frequencies, which modulate the electrostatic field by inducing a variation in capacitance between the stationary and moving elements responsive to relative motion of the elements, at modulation frequencies corresponding to the low and high spatial frequencies, substantially without spatial harmonics thereof; and
sensing the modulated electrostatic field and to determine responsive thereto coarse and fine measures of the position of the moving object.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a method for sensing the position of a moving object relative to a stationary object, including:
transmitting an electrostatic field in a vicinity of the moving object;
associating coarse and fine, periodic, electrically-active patterns with the moving object, the patterns varying along a dimension of motion of the object with respective low and high spatial frequencies, which modulate the electrostatic field by inducing a variation in capacitance between the stationary and moving elements responsive to relative motion of the elements, at modulation frequencies corresponding to the low and high spatial frequencies;
switching the electrostatic field so that it is modulated alternately by the coarse or by the fine pattern; and
sensing the modulated field so as to alternately determine, responsive thereto, coarse and fine measures of the position of the moving object.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, a method for sensing position of a rotating shaft, including:
coupling to the shaft a rotor with an electrically-active region which has a rotational asymmetry relative to an axis of the shaft and includes a pattern disposed circumferentially on the rotor, which pattern repeats multiple times about the shaft at a predetermined angular frequency;
transmitting an electrostatic field in a vicinity of the moving object;
sensing modulation of the field occurring once per rotation due to the rotational asymmetry of the region so as to determine, responsive thereto, a coarse measure of the rotation angle of the shaft; and
sensing modulation of the field due to the pattern so as to determine a fine measure of the rotation angle.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for sensing the position of a moving object relative to a stationary object, including:
coupling a moving element, having an electrically-active pattern thereon, to the moving object;
disposing first and second stationary elements on opposing sides of the moving element so as to transmit an electrostatic field therethrough;
sensing an electrical potential at the first stationary element and applying an opposite potential to the second stationary element so as to maintain the moving element at a generally constant potential; and
sensing modulation of the electrostatic field responsive to a variation in capacitance between the stationary elements due to movement of the electrically-active pattern therein so as to determine responsive thereto a measure of the position of the moving object.
There is likewise provided, in accordance with a preferred embodiment of the present invention, a method for sensing the position of a moving object relative to a curved surface, including:
fixing a reading head to the moving object;
fixing a flexible ruler along the curved surface;
providing an electrically-active pattern on the ruler or the reading head, which pattern causes a variation in capacitance between the ruler and the reading head so as to modulate the electrostatic field responsive to motion of the reading head relative to the ruler;
generating an electrostatic field in a vicinity of the reading head; and
sensing the modulated electrostatic field, so as to detect the modulation and determines responsive thereto a measure of the position of the moving object along the curved surface.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: